If we search for an m to demonstrate that k = 11 is not porous, we loop through all m where sum of digits = 11 and 11 a divisor of both m and rev(m). We find 209, 308, 407, ... 902, 2090, 3080, ... All m contain at least one zero. If it can be proven that this holds for all m, then 11 is a porous number.
For k = 11 this proof actually is quite easy:
Let "m_s ... m_3 m_2 m_1 m_0" be a number m with digits m_i and the sum of the digits is 11. We define:
A = m_0 + m_2 + m_4 + ... and B = m_1 + m_3 + m_5 + ...
A divisibility rule for 11 requires that the alternating sum of the digits must be a multiple of 11. Hence:
A - B = j * 11
Since the sum of the digits is 11, we have
A + B = 11
Adding the two equations yields
2 * A = (j + 1) * 11
Therefore A must be 0 or 11. If A is 11, then B is 0. This means either A or B must be zero and m must contain a zero. Hence 11 is a porous number.
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